Study shows modern city dwellers have lost about half of their gut microbes

Deep in the human gut, countless “good” bacteria and other microbes help us digest our food and keep us healthy by influencing our immune, metabolic, and nervous systems.

Some of these humble microbial helpers have been in our gut since before humans became human, and certain gut microbes are found in almost all primates, suggesting that they first settled in a common ancestor. But humans have also lost many of these helpers found in other primates, and may be losing even more as people around the world continue to flock to cities, one researcher reported last week at a microbiology conference in Washington, D.C. These absent gut microbes could affect human health.

The microbiome includes all bacteria, fungi, viruses and other microscopic life that inhabit individuals, whether human, plant or planarian. For humans and many other species, the most well-characterized microbiome centers on the bacteria in the gut. The more microbiologists study these gut microbes, the more they can relate these bacteria to the function of their hosts. For example, in humans, gut bacteria influence the immune system’s response to pathogens and allergens, or interact with the brain to influence mood.

Previous work by researchers has shown that human gut microbes are very similar to those of other primates, suggesting that their gut presence predates human evolution. But subsequent studies have also shown that, in a general sense, the human gut microbiome has become less diverse than the gut microbes of our current primate cousins. One study found 85 microbial genera, such as Bacteroides and Bifidobacterium, in the gut of wild apes, but only 55 in urban Americans and 60 to 65 of these bacterial groups in people in less developed parts of the world, an observation that links the decline in microbial diversity to urbanization.

As humans have moved from the hunter-gatherer past into cities, changes in diet, antibiotic use, more stressful living conditions, and better sanitation may have contributed to the loss of human gut microbes. Some prominent researchers believe that this lower diversity may have contributed to the increase in asthma and other inflammatory diseases.